(a) Field of the Invention
The present invention relates to a method of measuring, by utilizing ultrasound, the factor of stress concentration at the portion of a mechanical member, structural member or the like where stress is concentrated (will be referred to as "stress-concentrated portion" hereinafter).
The stress-concentrated portions of members, to which the present invention is applicable, include structural notched or cut portions such as hole, key way formed in members composing, for example, machineries, structures, etc. in all fields of industry, as well as undercut, blow hole, nest, crack, etc. developed in the manufacturing processes such as welding, forging, molding, etc. Also, they include the portions of materials, like flaw, crack, etc., of which the section varies abruptly.
More particularly, the present invention concerns a method of measuring the factor of stress concentration at the stress-concentrated portion of a member made of a metal or nonmetal (glass, ceramic, synthetic resin, etc.) and through which ultrasound can be propagated.
(b) Description of the prior art
Analysis concerning the stress-concentrated portion of a member, and setting of a factor of stress concentration at such portion, among others, are essential in designing and manufacturing a machine or structure in order to prevent any breakdown or damage of them and also to improve the safety and reliability. In the field of technology to which the present invention belongs, however, the method of measuring the factor of stress concentration of a real object easily, real-time and quantitatively is very important and necessary, but has not yet been established, for analysis of the stress concentration at the portions joined by the welding having been utilized from the old time and of which the application is very wide, and even for analysis of the stress concentration at the structural notched or cut portions of mechanical parts, typically, hole or key way formed by utilizing no welding and of which, it is said, the study has been highly advanced. It is very hard to theoretically analyze the mechanism of stress concentration even in these fields.
To solve the above problem, various methods of measuring the factor of stress concentration have been proposed. However, they include only the experimental methods of measuring using the photo-elasticity. That is, model experiments were made for analysis and study of only the typical models such as those in which circular holes or elliptical cavities were developed inside or on the surface of an elastic object like steel or in which U- or V-shaped notches were found on the surface of such object. These proposed methods were static, qualitative and indirect ones, and they were reported by H. Neuber and R. B. Heywood in 1958.
However, since the notches developed in the surface of bead weld vary in shape from one to another depending upon the method and kind of welding and also the shape of welded joint, the reproducibility can hardly be expected of the conventional method of measuring the factor of stress concentration at the weld zone. Therefore, it is practically impossible to condition any test piece of a weld zone into a predetermined model. In measurement of the factor of stress concentration at the welded portion, even the method of measuring by using the photo-elasticity, namely, a static, qualitative and indirect one, has some problems in applications and it is difficult to employ the method, as will be explained below.
(1) Although it is necessary to prepare a model as test piece, this modeling is impossible in practice for the above reasons.
(2) Even if such model could be prepared, since it is to be made from a material such as high-molecular epoxy resin, diarylphthalate resin or the like, differences in material, dimensions, working precision, etc. from the actual test piece are inevitably encountered which will also affect the fringe order of the stress-concentrated portion, resulting in no stable fringe order.
(3) Because of the material properties of the model, it is extremely difficult to make any acute-angle notch and so the reproduction of any stress concentration which will occur in the actual object is not expectable.
As a method of measuring the factor of stress concentration using no photo-elasticity, use of an electric resistance strain gauge (will be referred to as "strain gauge" hereinafter) is known; however, the strain gauge cannot be attached in any acute-angle notch. Even if the notch has an area wide enough to receive the strain gauge, the attaching of the strain gauge will cause of the stress-concentrated portion to have the properties changed. Thus, this method is also disadvantageous in impossibility of measuring any real factor of stress concentration. Namely, it is just an experimental method of measuring and has not been prevailing.
The present invention primarily seeks to provide a method of measuring, by utilizing ultrasound, the factor of stress concentration at the stress-concentrated portion of a mechanical or structural member, by which anyone can real-time measure the factor of stress concentration easily, quantitatively and with a high accuracy without changing the state in which the stress continuously acts on the stress-concentrated portion (will be referred to as "stressed state" hereinafter) and the nature of the portion in the stressed state.
Also the present invention seeks to provide a method of measuring the factor of stress concentration, which can measure the factor of a static stress concentration for an extremely short time (a few seconds) and also the factor of a dynamic stress concentration directly.
Furthermore, the present invention seeks to provide a method of measuring the factor of stress concentration, which can always provide for a highly accurate, quantitative measurement without being influenced by the shape and roughness of the surface on which the ultrasound probe is placed and even with more or less difference in placement of the probe.